The invention relates to an apparatus for producing an object using stereolithography, whereby layers of a liquid or powder material are formed and successively solidified.
A conventional stereolithographic apparatus is known from the German patent 41 34 265. This known apparatus comprises a wiper which may be provided with an antiadhesive coating or may be formed of a material having low adhesive properties. When using a wiper having low adhesion forces the contact between the wiper and the liquid material during wiping may locally be separated. This may cause the so-called "dewetting". On the other hand, if the wiper is formed of a material having a high surface tension, experience has shown that ununiform coatings occur because the material raises at the rear side of the wiper moving in wiping direction.
The DE 94 15 849 U1 discloses a stereolithographic apparatus with a wiper having at least two wiper members.
Various coating methods are used in stereolithography. According to EP 0 171 069 a liquid layer of the light curing resin is applied by lowering the object within a bath by an extent exceeding the desired layer thickness and subsequent raising the object to a level below the bath surface corresponding to the intended layer thickness. It is further known to apply the material for the layer from above using a material supply in EP 0 250 121 or a spray tube in WO 91/12120. The mentioned coating method do not yet provide an optimum speed of adjusting a desired layer thickness.
Coating methods using a wiper are known for accelerating the adjustment of the layer thickness. In the simplest case described in German patent 41 34 265 the support carrying the object to be formed is lowered by an extent corresponding to the desired layer thickness in a bath of light-curable liquid material, whereby unsolidified material flows onto the previously cured layer from the edges thereof. To accelerate the adjustment of the desired layer thickness the wiper strikes the material across the previously solidified layer. The coating process may also be accelerated by lowering the support in the bath by an extent exceeding the intended layer thickness, followed by striking with a wiper. However, it is the drawback of these methods that the produced layer thickness is considerably greater than the desired layer thickness. A solution to this problem known to the applicant consists in collecting the amount of resin which was displaced in the bath by lowering the support and continuously applying the same directly upstream of the wiper during the wiping process using a continuously operating low pressure pump, whereby a quasi continuous coating is obtained. A further known variation of a coating process is the so-called coating channel method wherein the amount of resin displaced by lowering the support is pumped in a single discrete pumping operation into a coating channel provided upstream of the wiper in wiping or stripping direction and delivered therefrom to a location directly upstream of the wiper through an outlet of the coating channel. The resin supply from the coating channel across the surface to be coated is an exponential function of the instantaneous resin level in the coating channel, i.e. the lower the resin level in the coating channel the less is supplied. Thus, the layer thickness decreases with the coating distance.
A general problem consisting in the use of conventional wipers is the interaction of the wiper with the used resin which may cause the above-mentioned "dewetting".
A further problem is shown in FIGS. 8 and 9. Within "closed volumes" 100, i.e. regions of liquid resin surrounded by solidified resin 101, the resin flowing around the wiper 99 causes the bath level to raise which results in dimensional divergences of the object, in particular of the object within the "closed volume" (CV+, CV-effects). A further disadvantageous effect of the known coating process using a wiper is a nose 102 which is shown in FIG. 10 and produced at the borderline between the previously solidified layer and the unsolidified resin. The flow of the resin around the wiper may also produce concave or convex surfaces which causes a reduced accuracy of the object.